1. Field of the Invention
The present invention relates to an embedded structure and the fabrication method thereof. In particular, the present invention relates to an embedded structure of circuit board with a smoother surface on the inner wall.
2. Description of the Prior Art
Circuit boards are essential elements in electronic devices. The function of the circuit boards is to define the pre-determined circuit patterns on a solid surface. For the trend of miniaturization of the electronic devices, the line width and the line space of the conductive wires on the circuit boards are narrower and narrower.
Currently, there are two methods available to form the circuit boards to meet the demand. The first one is to transfer-print patterned wires into a dielectric layer. The other one is to pattern a substrate by laser to define a damascene structure, then use a conductive material to fill the recesses on the substrate to complete an embedded structure. Generally speaking, the surface of the substrate is required to be activated in advance to allow the conductive material to successfully fill the recesses on the substrate, frequently by electroless plating methods. Even more, a material is proposed to allow the conductive material to be disposed on the recesses on the substrate without a prior activation procedure for the electroless plating.
FIGS. 1-4 illustrate a conventional method to form an embedded structure. As shown in FIG. 1, first, a substrate 101 is provided. A first patterned copper layer 110 is disposed on the substrate 101 and partially exposes the substrate 101. A first dielectric layer 120 covers the first patterned copper layer 110 and the substrate 101.
As shown in FIG. 2, the first dielectric layer 120 is patterned to form a pad opening 122, a via 121 integrally formed with the pad opening 122, and a trench 123 adjacent to the pad opening 122. The via 121 exposes a portion of the first patterned copper layer 110. Because residues may remain on the exposed surface of the first patterned copper layer 110 and would hinder the following electric connection, a desmearing procedure is carried out, as shown in FIG. 3 to remove the residues on the exposed surface of the first patterned copper layer 110 and to facilitate the following electric connection. The desmearing procedure may be carried out by employing plasma or an oxidizing agent, such as permanganate. In addition to removing all the residues on the exposed surface of the first patterned copper layer 110, the desmearing procedure also etches the surface of the first dielectric layer 120, including sidewalls of the pad opening 122, the via 122 as well as the trench 123, thereby forming a rugged surface on the first dielectric layer 120. If such rugged surface further undergoes a copper deposition, as shown in FIG. 4, undesirable lumps 131 occur everywhere in the copper layer 130 and deteriorate the quality of the copper layer 130, or the rugged surface limits the design of circuit of high density. Even more, the rugged surface of the trench 123 results in rugged circuits and causes signal loss. The copper layer 130 of bad quality jeopardizes the reliability of the embedded structure 100, of the circuit board and of the electric device made thereof. It is a disadvantage to be overcome.
Therefore, an embedded structure of better surface evenness as well as a novel manufacturing process are needed to provide a circuit board with good reliability.